One form of charge-coupled device (CCD) comprises a silicon die that has been processed using conventional MOS technology to form a buried channel beneath its front surface (the surface through which the die is processed). The channel is made up of a linear array of like elementary regions, and each region has a well defined potential profile including several, e. g. three, potential levels of controlled potential depth. A clocking electrode structure overlies the front surface of the die, and by application of selected potentials to the clocking electrode structure, charge present in a given elementary region may be advanced through the linear array of elementary regions, in the manner of a shift register, and extracted from the channel at an output electrode. Charge may be introduced into the channel at an input electrode that is at the opposite end of the channel from the output electrode, or may be generated photoelectrically. Thus, if electromagnetic radiation is incident on the substrate beneath the channel layer it may cause generation of conduction electrons and these conduction electrons may enter the channel layer and become trapped in a potential well defined between two zones at higher potentials. The diffusion length of the conduction electrons is sufficiently short that a conduction electron generated in the substrate will not pass by diffusion farther than the elementary channel region that immediately overlies the substrate region in which the conduction electron was generated.
A CCD may be constructed with a plurality of parallel buried channels. One application of such a multiple channel CCD, using photoelectrically generated conduction electrons, is as a solid state imager or opto-electric transducer. The die in which the CCD is fabricated is thinned from its back surface, so as to bring the back surface as close as possible to the channel layer, and the die is placed with its back surface at the focal plane of a camera so that the camera lens forms an image of a scene on the back surface of the die. The CCD may comprise, e. g. 512 parallel channels each having 512 elementary regions and the resulting 512 .times.512 array of elementary regions resolves the back, or image receiving, surface of the die into 512.times.512 picture elements or pixels. The intensity of the optical energy incident on a given pixel can influence the electron population of the associated elementary region of the channel layer, and so the number of electrons that are transferred out of an elementary region, and ultimately extracted from the CCD, indicate the intensity of the light incident on the pixel. By controlling the timing of the clock pulses in relation to the illumination of the CCD, the CCD can be used to generate an electrical signal representative of the distribution of light intensity over the image receiving surface of the CCD, i. e. of the image formed by the camera lens.
The interface between the channel layer and the substrate of the die is located at a physical depth beneath the first surface of the die in the range from about 5 .mu.m to about 150 .mu.m and therefore in order to maximize the diffusion of photoelectrically-generated conduction electrons into the channel layer it is desirable that the thickness of the silicon die be in the range from about 10 .mu.m to about 160 .mu.m. Since an unprocessed silicon wafer normally has a thickness of about 1 mm because it must be sufficiently thick to be self-supporting during processing, this implies that the wafer must be thinned in order to produce a die having a thickness of 160 .mu.m or less.
It is conventional to carry out thinning of a silicon wafer by grinding or etching. .However, in order to thin a wafer to less than about 250 .mu.m, it is necessary to provide the wafer with a mechanical support layer, and it has hitherto been conventional to use a wax support layer. The wax is applied in molten form to the front surface of the wafer, and the wafer is thinned from its back surface. However, waxes that have conventionally been used do not adhere to the wafer particularly well, and therefore when the wafer is thinned to 160 .mu.m or less the silicon die frequently peels from the wax layer and disintegrates. Even if the die does not disintegrate, it may suffer local detachment from the wax layer, with the result that the back surface of the die ceases to be planar, and this may be unacceptable.
In order to minimize dark current generated in the CCD, it is desirable to operate the CCD at a depressed temperature. In order to achieve this, it is conventional to cool the CCD with liquid nitrogen. At standard pressure, liquid nitrogen evaporates at a temperature of about -196 degrees C. Therefore, it may be necessary for a CCD imager to be capable of withstanding temperature changes of over 200 degrees C. This implies that there must be a good matching of the coefficients of thermal expansion between the silicon die and the mechanical support layer, since otherwise differential expansion and contraction will damage the silicon die.
It is not essential that there be a perfect match between the expansion coefficients of the silicon die and the support layer, since some bowing of the back surface of the die can be tolerated. However, the degree of bowing that can be tolerated is quite small, particularly if application of the support layer involves subjecting the wafer to an elevated temperature, so that the wafer is bowed upon cooling to ambient temperature, since it is difficult to thin a bowed wafer.
In U.S. Pat. No. 4,422,091 it is proposed that a CCD imager that has been fabricated using heteroepitaxial gallium arsenide technology should be supported during thinning by means of a plate of molybdenum, aluminum or glass that is bonded to the die using epoxy adhesive or a bonding alloy, for example. U.S. Pat. No. 4,422,091 also proposes that the CCD die be supported during thinning using a GaAs or Si chip that has its own signal conditioning and amplification circuit incorporated therein.